TCDD (dioxin, 2,3,7,8-tetrachlorodibenzo-o-dioxin) is one of the most toxic chemicals known to man. Progressive weight loss and thymic atrophy are two of its most frequent toxic symptoms. The induction of cleft palate and hydronephrosis characterize the teratogenic response of mice to TCDD. Because of the sensitivity of this response, we decided to use teratogenicity to measure the interaction of TCDD and other compounds with which it occurs in the environment. Such chemicals include polychlorinated dibenzofurans, polychlorinated biophenyls, hormones such as thyroxins and hydrocortisone, and drugs. TCDD interacts in an additive manner with 2,3,7,8-tetrachlorodibenzofuran and 2,3,3',4,4',5-hexachlorobiphenyl, synergistically with hydrocortisone. Its teratogenicity is potentiated by thyroid hormones. Polychlorinated dibenzofurans cause the same spectrum of teratogenic effects as TCDD and appear to interact additively with each other.